This invention relates to solid state, nuclear radiation detectors. More specifically, this invention relates to germanium radiation detectors wherein deep p-n junctions are produced by the diffusion of lithium.
Germanium devices for the detection of gamma radiation are well known. Typically, gamma radiation is caused to impinge on a germanium body at 77.degree. K. A p-n junction is formed within the germanium and is reverse biased with an electric field to deplete charge carriers from a volume adjoining the junction. Gamma rays impinging on the depleted region excite charge carriers and produce a current pulse across the junction.
It is desirable to produce germanium detectors having large electric fields within the depletion region. Such fields increase the drift velocity of charge carriers within the detector and thereby improve the detector pulse response time. High fields also minimize the effects of charge carrier trapping within the depletion region.
It is generally undesirable, however, to operate germanium detectors with high electric fields in the surface or contact regions. High surface fields contribute to detector noise and increase the probability of breakdown between contacts. Prior art germanium radiation detectors have generally been produced by a lithium drifting process which is described, for example, in U.S. Pat. No. 3,016,313 to Pell. A high concentration of lithium is diffused into the surface of a body of p-type germanium to form a narrow p-n junction. The germanium is heated while a strong, reversed bias electric field is applied across the junction to drift the lithium through the structure. The electric fields in coaxial lithium drifted germanium detectors reach their highest values at the radius of the inner electrode. Drifted detectors must, generally, be stored and operated at cryogenic temperatures to prevent precipitation of the highly concentrated lithium dopant.
Germanium detectors have also been formed by producing a p-n junction at one surface of a body of high purity p-type germanium, for example, those described in Planar and Coaxial High Purity Germanium Radiation Detectors, J. Llacer, Nuclear Instruments and Methods, 98 (1972), 259-268. Detectors of this type are characterized by high electric field concentrations in the region of the negative contact.
Prior art methods, for example diffusion or ion implantation, are incapable of producing a junction deep within a germanium body.
Prior art planar detectors have also been produced by utilizing naturally occurring junctions within bodies of high purity germanium, for example, those described in A Large Volume High Purity Germanium Radiation Detector, J. Llacer, Nuclear Instruments and Methods, 104 (1972), 249-251. The geometry of such detectors is, however, limited by the occurrence and shape of the natureal p-n junctions which occur in a random and unpredictable manner during germanium crystal growth.